Ion beams come in many varieties and have many industrial applications. For example, a collimated low-power ion beam may be employed to align inorganic materials in a liquid crystal display. Alternatively, a collimated high-power ion beam may be employed to ion etch a surface or propel a vehicle in space. A highly collimated ion beam, which has minimum off-axis velocity components, is generally desirable in many broad beam ion source applications. Other applications of collimated ion beams include without limitation ion milling and aligning superconducting YBa2Cu3O7 crystals as they grow using Ion Beam Assisted Deposition techniques to increase the current carrying capability of High Temperature Superconducting tapes.
An “ion” is a charged particle, referring to any atom or molecule having an unbalanced electrical charge, which is a result of having lost or gained one or more electrons. A gas phase collection of ionized atoms, molecules, and/or electrons is referred to as “plasma”. Generally, plasma states can be induced in gases, such as neon, argon, etc., by applying high-energy radio frequency (RF) fields or a direct current (DC) voltage to the gaseous matter. The radio frequency fields remove electrons from the particles, giving the particles a positive charge. Using electric fields, the ions can then be extracted from the plasma and propelled to a target in an ion beam.
One technique for extracting ions from the plasma involves placing an electrical field near the plasma. A simple example of an ion beam provides plasma in a plasma chamber with two electrified grids positioned at an opening of the plasma chamber. Each grid has an array of apertures to allow ions to travel through the grid during operation. Typically, the apertures of one grid are closely aligned with apertures of the other grid. The first electrified grid (i.e., the grid closest to the plasma) is called an “extraction grid” or a “screen grid”, and has a high positive electrical potential (e.g., 1000 Volts). The second grid, called an “acceleration grid”, is spaced closely to the first grid and has a negative potential (e.g., −400 Volts).
For 2-grid systems, ion beam divergence is strongly dependent upon normalized perveance per aperture, the extraction-grid-to-acceleration-grid spacing, the aperture size, and the net-to-total-accelerating-potential ratio. Perveance is a normalized measure of the current of ions extracted through each aperture. Adjustments to the spacing and aperture hole sizes can reduce the net ions impinging upon the grids and decrease the angular divergence of the ion beam. It has been shown that the best-case divergence angle for a two-grid ion source is 10 degrees and can be as large as 30 degrees. See G. Aston et al. “Ion Beam Divergence Characteristics of Two-Grid Accelerator Systems”, AIAA Journal, Vol. 16, No. 5, May 1978, pp. 516–524; G. Aston et al., “The ion-optics of a Two-Grid Electron Bombardment Thruster,” AIAA Paper 76–1029, Key Biscayne, Fla., 1976; Y. Hayakawa et al., “Ion Beamlet Divergence Characteristics of Two-Grid Multiple-Hole Ion Accelerator Systems,” AIAA Paper 97–3195, Seattle, Wash., 1997.
Another technique involves a third grid, called a “shield grid”, which is placed the furthest away from the plasma chamber and is typically spaced closely to the acceleration grid. In many applications, an RF excited plasma bridge neutralizer is positioned in the vicinity of the ion beam output and is used to provide electrons for current and space charge neutralization of the ion beam, for example, to reduce inter-ion repulsion within the ion beam. The shield grid is typically charged to a low electrical potential (e.g., 0 Volts). By positioning the shield grid downstream (i.e., away from the plasma source) of the acceleration grid and operating it at a near ground potential, the neutralization plane becomes fixed in close proximity to the shield grid potential. This characteristic allows for flatter equipotential surfaces between the neutralization plane and the acceleration grid apertures as compared to the 2-grid system, resulting in less off axis divergence of the ion beam (e.g., the best case is about 8.2 degrees).
However, some applications require less off-axis divergence. In an attempt to decrease off-axis divergence, one existing technique has introduced a second positive electrical potential acceleration grid between the extraction grid and the negative-potential acceleration grid described previously. See PCT Application PCT/GB97/02923, ION GUN, by Nordiko Ltd., published 30 Apr. 1998. The new acceleration grid is intended to provide “gentler” acceleration so as to allow formation of a lower current, more stably collimated beam, which is less susceptible to space charge forces. However, the existing 4-grid optics system requires the new acceleration grid to be contoured (instead of flat), resulting in non-uniform spacing from center-to-edge between the grids in the ion beam source. Moreover, the main result of “gentle” extraction of the ions by the existing 4-grid optics system is low perveance and, therefore, a reduced ion beam current caused by the reduced electrical potential between the extraction grid and the new acceleration grid, which is predicted by Child's Law. See Child, C. D., Physical Review, Vol. 32, 1911, pp. 492–511. No experimental data is available to support the claim of improved collimation in any existing 4-grid optics system and, furthermore, the reduced ion beam current is inadequate for many applications. Accordingly, existing ion beam systems fail to provide adequate collimation and ion beam current for many applications.